• If the negative sequence current contributions from the W1 and the W2 sides are
in phase, the fault is internal
• If the negative sequence currents contributions from W1 and W2 sides are 180
degrees out of phase, the fault is external
For example, for any unsymmetrical external fault, ideally the respective negative
sequence current contributions from the W1 and W2 power transformer sides will be
exactly 180 degrees apart and equal in magnitude. An example is shown in figure
34,
which shows trajectories of the two separate phasors representing the negative
sequence current contributions from the HV and LV sides of an Yd5 power
transformer (after compensation of the transformer turns ratio and phase
displacement) for an unsymmetrical external fault. Observe that the relative phase
angle between these two phasors is 180 electrical degrees at any point in time. No
current transformer saturation was assumed for this case.
en05000189.vsd
0.1 kA
30
210
60
240
90
270
150
330
180 0
0.2 kA
0.3 kA
0.4 kA
"steady state"
for HV side
neg. seq. phasor
10
ms
10
ms
"steady state"
for LV side
neg. seq. phasor
Contribution to neg. seq. differential current from HV side
Contribution to neg. seq. differential current from LV side
IEC05000189 V1 EN
Figure 34: Trajectories of Negative Sequence Current Contributions from HV
and LV sides of Yd5 power transformer during external fault
Under external fault conditions, the relative angle between the phasors is theoretically
equal to 180 degrees. During internal faults, the angle shall ideally be 0 degrees, but
due to possible different negative sequence source impedance angles on the W1 and
W2 sides of the protected power transformer, it may differ somewhat from the ideal
1MRK 502 048-UEN A Section 6
Differential protection
99
Technical manual
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